Halo around Alnitak: regular filter behaviour or something else?

[freiform]

Hi,

my Ha exposures show a relatively pronounced halo around Alnitak. The OIII also shows this but not as heavily and the SII looks fine. Is this regular behaviour for ZWO 31mm filters (new version)?
Or have I mounted the filter(s?) the wrong way around? I wasn't sure when equipping the wheel because I couldn't quite reproduce the hue given on the ZWO site [1]. And so far I didn't have such bright stars in my captures so the issue did not show.

Also, I suppose the halo not beeing concentric hints at a tilted sensor or filter?

Any insight would be greatly appreciated.

Sven

 

(Single frame, 120s Ha, cropped )

 

[1]https://astronomy-imaging-camera.com/product/zwo-new-narrowband-31mm-filter

Edited October 31, 2019 by freiform

[Adam J]

On Alnitak nothing but a premium filter will result in no reflections. So its within what I would expect for filters in their price category. Depending on if its the V1 or V2 ZWO filters it will be better or worse.  As you have only two reflections visible I would say these are the updated ones, if it had been V1 then you would have had 6 or more rings. None concentric reflections of this type are normally a sign that the filter is slightly tilted, its not too bad though. 

[Yawning Angel]

For comparison, this is with the older ZWO filters

[freiform]

Adam, Alex, thanks!

That is reassuring, I guess. Buf of course the price of the product is (well, mostly, hopefully) reflected in its performance (no pun intended..).

Out of curiosity; how would a image look like with the filter mounted the wrong way? I probably will have the last few hours of clear skies for teh next few weeks so I'd rather be spending the time capturing actual data 

 

Sven

[Adam J]

20 minutes ago, freiform said:

Adam, Alex, thanks!

That is reassuring, I guess. Buf of course the price of the product is (well, mostly, hopefully) reflected in its performance (no pun intended..).

Out of curiosity; how would a image look like with the filter mounted the wrong way? I probably will have the last few hours of clear skies for teh next few weeks so I'd rather be spending the time capturing actual data 

 

Sven

There are different mechanics for filter reflections, the above are reflection internal to the filter. I would expect those would be unaffected by reversing the filter. The other type are reflections between the filter and the camera surface or the filter and another optical element, the spacing is larger so the size of the reflections is bigger and more diffuse, its that type of reflection that has a dependency on filter direction. 

[steppenwolf]

Your filters will work correctly as 'filters' mounted either way round but if you examine the reflections from each filter and note that one side is more 'reflective' than the other, there is an argument for placing the higher reflectance surface on the telescope side rather than the camera side to avoid reflections between the sensor chamber glass and the filter. BUT (there's always a but....), this does increase the risk of reflecting light onto a nearby field flattener or focal reducer which could be a further source of halos!

The surest way is to take some test images in both orientations and decide on the one that suits your filter/camera/corrector the best, assuming that your filter wheel will allow both orientations in the first place without anything touching the housing/disk mounting- some filter wheels most likely will not.

[freiform]

Thank you for the replies. That clearifies things.

 

I will do some test images on bright targets if I ever get clear skies again. Alternatively, an artifical star should do the trick as well I suppose.

 

Sven